However if the flash is off-camera, step 3 is no longer straight-forward. I may have placed the flash on my scene in a position where it is impossible to obtain a proper exposure no matter the power of the flash.
In this case, however, TTL is not the issue, and the situation would not be fixed by putting the flash in M. This is an issue about the maximum power output of the flash and may only be solvable by getting a studio strobe. :) This is not a test of the usefulness/practically of off-camera TTL.
As a quick example, let's say that I want to use the flash as a back-light for a portrait. ...So if the flash assumes that it needs a lot of power to obtain the proper exposure (because let's say that the face of my model is really underexposed) its batteries will be wasted as it goes to full power since 99% of its light never changes the picture. ... Is my assumption correct?
Yes. And this is why today's radio triggering systems offer you the ability to mix M and TTL groups together in a single setup. For certain functions, such as background (where you would likely want to over/underexposed based on TTL anyway) or rim lights, you probably want to set those groups up as M, but key and fill are a different story.
Or does TTL behave differently when on-camera and off-camera?
It does or it doesn't, depending on how you look at it and how TTL is being performed. Because on-camera flash can only be a single light in a single group, TTL is going to be a simpler scenario.
With off-camera flash, system implementation of TTL for multiple groups can be different. Some do the simplest scenario where all flashes fire at the same time for the pre-flash, a single measurement is taken for the whole scene, and then one single power setting is pushed out to all the flashes (with or without FEC). This is how Canon's A+B+C ratio setting works, and it's the only mode that Godox uses for multiple-group TTL setting. In this type of usage scenario, as long as you're happy for all the individual group settings to be based on the same reading, you'll be fine. But it's most likely you'll only be happy using TTL for a single group of lights, or if all your TTL groups are matched (same light) and roughly the same distance from your subject.
But in some systems, lights fire individual pre-flashes for measurement by group, and TTL sets the power level individually per group, remaining more accurate per-light. This is more likely to be useful for more accurate lighting ratios with lights that are mismatched or at different distances from the subject.
The advantages to using TTL for off-camera lights are that TTL makes changes to ISO, aperture, and distance transparent to the flash exposure as long as the power level required is within the flash's power range. You can flip from f/1.4 to f/5.6 to f/2.8; or iso 200 to 800 to 400; or move your lights out a foot or in a meter and TTL can adjust the flash's power automatically for you so the flash exposure remains the same for each shot. You can flow more dynamically through your setup or from one setup to another than if you have to keep adjusting the light manually for every change to these three factors.
If you're a macro shooter that likes to use the lights in close, another advantage to using TTL with off-camera flash is that you can go below the minimum power level.
The main disadvantages of TTL are that explicit control over more than two groups gets messy. And shot-to-shot inconsistency happens, because changing anything in the frame can change metering and then the lights will also be adjusted.
This latter issue, however, has been addressed in newer wireless flash triggering system with a feature I call "TTL locking": a way to lock the TTL-set power level by translating it into an M power setting. Everyone has a different name for this feature:
- Profoto: (not really a name) just "switch to Manual"
- Godox: TCM (TTL Convert to Manual)
- Westcott/Jinbei: Equivalent Manual Exposure
- Cactus: Flash Power Lock
- Nissin: TTL to Manual Conversion
- Canon: FE Memory [FE = flash exposure]
- Sony: Memory Level
But this feature was first introduced by Profoto around 2015. Godox and Cactus followed suit around 2018, and Canon only added it in 2021 with their ST-E3-RT v2. So it's a relatively new feature that requires newer gear, and is therefore not something a lot of folks know about, let alone use, let alone teach at this time.
TTL is very useful so you don't have to lock down iso, aperture, and light placement for a shoot. But mostly only for the key/fill lights in TTL, with lights that are matched and roughly the same distance from the subject or only the key light. And is best done with a triggering system that supports mixed TTL and M groups and TTL locking.